Bash variable substitution vs dirname and basename

by

The external commands make some logical corrections. Check the result of the next script:

doit() {
    str=$1
    echo -e "string   $str"
    cmd=basename
    [[ "${str##*/}" == "$($cmd $str)" ]] && echo "$cmd same: ${str##*/}" || echo -e "$cmd different \${str##*/}\t>${str##*/}<\tvs command:\t>$($cmd $str)<"
    cmd=dirname
    [[ "${str%/*}"  == "$($cmd $str)" ]] && echo "$cmd  same: ${str%/*}" || echo -e "$cmd  different \${str%/*}\t>${str%/*}<\tvs command:\t>$($cmd $str)<"
    echo
}

doit /aaa/bbb/
doit /
doit /aaa
doit aaa
doit aaa/
doit aaa/xxx

with the result

string   /aaa/bbb/
basename different ${str##*/}   ><          vs command: >bbb<
dirname  different ${str%/*}    >/aaa/bbb<  vs command: >/aaa<

string   /
basename different ${str##*/}   ><  vs command: >/<
dirname  different ${str%/*}    ><  vs command: >/<

string   /aaa
basename same: aaa
dirname  different ${str%/*}    ><  vs command: >/<

string   aaa
basename same: aaa
dirname  different ${str%/*}    >aaa<   vs command: >.<

string   aaa/
basename different ${str##*/}   ><  vs command: >aaa<
dirname  different ${str%/*}    >aaa<   vs command: >.<

string   aaa/xxx
basename same: xxx
dirname  same: aaa

One of most interesting results is the $(dirname "aaa"). The external command dirname correctly returns . but the variable expansion ${str%/*} returns the incorrect value aaa.

Alternative presentation

Script:

doit() {
    strings=( "[[$1]]"
    "[[$(basename "$1")]]"
    "[[${1##*/}]]"
    "[[$(dirname "$1")]]"
    "[[${1%/*}]]" )
    printf "%-15s %-15s %-15s %-15s %-15s\n" "${strings[@]}"
}


printf "%-15s %-15s %-15s %-15s %-15s\n" \
    'file' 'basename $file' '${file##*/}' 'dirname $file' '${file%/*}'

doit /aaa/bbb/
doit /
doit /aaa
doit aaa
doit aaa/
doit aaa/xxx
doit aaa//

Output:

file            basename $file  ${file##*/}     dirname $file   ${file%/*}     
[[/aaa/bbb/]]   [[bbb]]         [[]]            [[/aaa]]        [[/aaa/bbb]]   
[[/]]           [[/]]           [[]]            [[/]]           [[]]           
[[/aaa]]        [[aaa]]         [[aaa]]         [[/]]           [[]]           
[[aaa]]         [[aaa]]         [[aaa]]         [[.]]           [[aaa]]        
[[aaa/]]        [[aaa]]         [[]]            [[.]]           [[aaa]]        
[[aaa/xxx]]     [[xxx]]         [[xxx]]         [[aaa]]         [[aaa]]        
[[aaa//]]       [[aaa]]         [[]]            [[.]]           [[aaa/]]

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